1. Field of the Invention
The present invention relates generally to a printing apparatus and process, and is particularly concerned with an apparatus and process for printing date codes and other types of identifying indicia on the surfaces of webs or discrete articles carried by an intermittently moving conveyor.
2. Description of the Prior Art
In many types of product manufacturing, packaging, and handling operations it is necessary to print some sort of identifying indicia on the products in addition to the usual pre-printed labels, packaging material and so on. In some cases the required indicia consists of a number or code signifying something about the product, such as a unit price, or the process used to manufacture the product, or perhaps the particular place dwhere the product was manufactured. More frequently, as in the case of perishable food products or pharmaceuticals, the indicia consists of a date code, signifying either the date of manufacture or the last date on which the product can be sold or used. These dates are quite familiar to the consumer as the expiration dates which commonly appear on containers for dairy products, medicines, and other products with a limited shelf life.
Since the date to be printed will usually be changed very frequently, as for example daily, it is impractical to pre-print this information on boxes, labels or other previously prepared types of packaging material. For this reason, various types of printing or marking devices have been developed for rapidly and efficiently printing date codes and similar types of information on products or product containers at some point during the manufacturing or packaging operation. For example, the date codes may be printed in blank locations on a continuous web of pre-printed product wrappers or labels before the individual wrappers or labels are cut from the web and applied to the products. In other cases, the date codes may be printed on the products or product containers after all of the various manufacturing, packaging and labeling operations have been completed.
A common prior art technique for carrying out rapid printing or marking of webs or discrete articles is to provide the printing member in the form of a rotating element, such as a rotating die roll, which carries a printing die or other type of printing device on its peripheral surface. In this type of system, articles transported by a moving conveyer system, or discrete sections of a moving web, are successively brought into contact with the moving printing die after the latter has been inked by a suitable inking mechanism. This approach, which depends essentially upon rolling contact between the printing die and the surface being marked, obviously requires that the conveyer be kept in motion, preferably continuously but at least during the time that the printing die is in contact with the article or web surface. In either case, great care must be taken to equalize the peripheral velocity of the printing die with the velocity of the surface to be printed, since blurring of the printed image can occur if these velocities differ substantially. Another difficulty with this type of system is that, if the printing member and the article or web conveyor are to operate at fixed velocities, the spacing between adjacent articles must be equal to the spacing between adjacent printing dies on the die roll, which is an undesirable constraint in some instances. Measures can be taken to avoid this limitation, such as by moving either the die roll or the conveyor intermittently rather than continuously. Although intermittent motion of the die roll or conveyor is an effective means for avoiding the spacing limitation, there is a concomitant increase in the complexity of the system. In addition, with repeated acceleration and deceleration of the die roll or conveyor, the necessary equality between the article or web velocity and the peripheral velocity of the die wheel during printing becomes more difficult to achieve.
Other types of prior art marking devices have utilized reciprocating or stamping mechanisms for the printing member, as distinguished from the rotary-type printing systems described earlier. Since reciprocating or stamping mechanisms are generally intended to print on stationary surfaces, they are often preferred in cases where an intermittently moving conveyor is used to transport the articles or web to be printed. For example, the printing operation may be preceded or followed by another operation requiring temporary stopping of the conveyor motion, such as a container filling or sealing operation, and in these instances intermittent movement of the conveyor may be unavoidable. In situations like these, the temporary stopping of the conveyor provides a convenient interval during which printing can be carried out on the article or web surface by a reciprocating or stamping mechanism, with inking of the printing die occurring either at some point during the printing cycle or between successive printing cycles.
The inking of the printing die usually presents somewhat more of a problem in reciprocating or stamping mechanisms than it does in simple rotary printing systems. In the latter type of system it is merely necessary to position an inking device in a fixed location that is tangential to the rotary path of the printing die, which will suffice to apply ink to the die once during each full rotation or cycle of the printing member. In reciprocating or stamping type systems, however, the printing member generally moves in a back-and-forth manner along a path which may consist of a straight line, an arc, or some sequence or combination thereof. If the position of the inking device were to be fixed relative to the path of the printing member, the printing die would contact the inking device twice during each printing cycle, once during the forward movement of the printing member and again during its reverse or return movement. This can present difficulties in cases where the inking device consists of an inking roller which is arranged for powered rotation in only one direction by a motor, gear train, or the like, since smooth rolling contact between the printing die and the periphery of the inking roll is possible only when both are moving in the same direction. An additional problem is that at least a portion of the path of the printing member is usually in a direction normal to the plane of the printing die, which makes it difficult to position the inking device so that it will properly apply ink to the surface of the printing die without physically obstructing the movement of the printing member.
For these reasons, various types of movably mounted inking devices have been proposed for use in reciprocating or stamping type article printing systems. Generally, the movement of the inking device is such that inking occurs only once during each cycle of movement of the printing member, with the inking device being withdrawn to a non-inerfering position relative to the printing member during the remainder of the printing cycle. For example, it is common to provide a movable inking pad or roller which is arranged to be swung into momentary contact with a printing member while the latter is temporarily held stationary at an inoperative or non-printing position. In an alternative arrangement, a movable inking roller is oscillated between a retracted position within a fixed ink reservoir and an extended or operative position in the path of a moving printing member, with the movement of the inking roller being such that ink is applied to the printing die only during the forward stroke of the printing member toward the article to be printed. This arrangement differs from the previously-described systems of swinging ink pads or rollers in that the ink is applied to the printing die while the printing member is moving in the direction of the article to be printed, rather than at a stationary point of the printing member.
Still another approach to the problem of applying ink to the printing die in a reciprocating or stamping type system is to provide the die carrying member with an arrangement for alternately raising and lowering the printing die as the die carrying member moves back and forth between an inking roll and an article to be printed, thereby ensuring that the die is inked only once during the printing cycle. In other words, an additional motion of the printing member is utilized in order to allow the inking device to remain in a fixed position. This arrangement does effectively avoid the problems referred to above, but only at the expense of greater complexity in the design of the die carrying member to achieve the desired motion of the die relative to the inking roll.
Another problem that is encountered with article marking systems, or indeed with any type of printing system in which ink must be applied to a printing element such as a printing die or a row of type, is that of obtaining uniform application of ink to the entire surface of the printing element. This problem is particularly troublesome where the printing element is substantially flat, and where the inking device is in the form of a cylindrical roller which is intended to be brought into tangential rolling or wiping contact with the surface of the printing element as the latter is moved in an arcuate path by a pivoting printing arm, die roll, or the like. It is intuitively apparent that a flat printing element, moving in an arcuate path about a fixed axis, cannot be maintained in uniform contact with the periphery of a fixed inking roll as the line of contact between the two moves across the surface of the printing element. On the contrary, since the leading and trailing edges of the printing element are effectively at a greater radius from the axis of rotation of the printing arm or die roll than the median line of the printing element, the contact pressure between the printing element and the inking roll will gradually decrease as the line of contact between the two moves from the leading edge of the printing element to the median line, perhaps to the point where the printing element and inking roll will begin to physically separate, and will then gradually increase as the line of contact progresses toward the trailing edge of the printing element. As a result, if the contact pressure is set to the desired amount at the leading and trailing edges of the printing element, there will be insufficient contact pressure at the median line of the printing element. Conversely, if the contact pressure is set to the desired amount at the median line of the printing element, the contact pressure will be excessive at the leading and trailing edges of the printing element. In either case, the result is usually nonuniform inking of the printine element.
The prior art approach to the problem of nonuniform inking has usually been to limit the size of the printing die or type row relative to its radius of rotation, so that the nonuniformity in contact pressure, while still present, is at least minimized. Another approach is to provide the inking roll and/or the printing element with a resilient surface in order to maintain at least some degree of contact between the two despite variations in the contact pressure. Neither of these approaches has proved to be entirely satisfactory. Still another approach is to provide the printing element with a slight curvature, with the radius of curvature corresponding to the effective radius of the pivoting printing arm or die roll. Although this is indeed effective to alleviate the problem of nonuniform inking, curved printing dies are more difficult to fabricate than flat dies and can only be used with a printing arm or die roll having the proper radius. In the case of rotary-type printing devices employing flat printing dies, a compensation system has been devised wherein the rotation of the die-carrying member is controlled by a fixed cam which gradually shifts the axis of rotation to the die-carrying member to assure uniform contact between the printing die and adjacent inking and offset rolls. However, this type of compensation system is not conveniently applicable to reciprocating or stamping type printing mechanisms, since it involves adding additional complexity to a printing member which may already be required to move in a complicated curvilinear path between the inking device and the article to be printed.
To be commercially acceptable, product marking devices are required to be relatively inexpensive, simple to repair and maintain, and above all, reliable and trouble-free. The requirements of serviceability and reliability, in particular, are readily understood when it is realized that a product marking unit will typically be used by a product manufacturer who may have little or no understanding of its construction or operation, but who will stand to lose a great deal if a malfunction in the product marking unit were to force a temporary shutdown of the entire product manufacturing operation. This kind of occurrence must, of course, be avoided at all costs. By and large, the product marking machines to be found in the prior art are characterized either by overly complex mechanisms which are prone to failure, or, on the other hand, by mechanisms which, although simple in construction, are lacking in features necessary to assure proper and efficient operation, such as compensating arrangements for assuring uniform application of ink to flat printing elements.